1. Field of the Invention
The present invention relates to a process for metalizing a substrate, and more particularly to an electrolytic process for metallization of a substrate and an electrolytically metallized article manufactured thereby.
2. Background of the Art
For various uses, substrate materials of a metallic or nonmetallic nature are provided with metal coatings to improve their physical and/or chemical characteristics. A particularly economical metal coating process suitable for this purpose is electrolytic deposition. This can generally be accomplished directly on metal substrates after cleaning same and on nonmetal substrates, i.e. on substrates which are poor conductors or are non-conductors of electricity, after a premetallization thereof by physical or chemical techniques, for example, by vacuum deposition or electroless deposition, respectively.
During further processing or during practical use, these electrolytically produced metal layers must be able to withstand a series of different stresses. For example, thermal stresses must not unduly change the adhesion characteristics of the metal layer to the substrate. Adhesion of a metal layer is often undermined by critical loads from various sources of stresses, such as the mechanical stresses between the metal layer and the substrate which accompany a temperature change and are due to differing coefficients of thermal expansion and/or to chemical changes between the metal layer and the substrate material.
Electrolytic deposition in particular is known to be accompanied by inclusion of unwanted volatile or volatilizable substances, such as hydrogen, water, electrolyte residues, and organic plating bath additives, etc., in the metal layer and/or at the interface between the metal layer and the substrate, particularly if the substrates are porous. Thermal treatment of a metal layer with included impurities may generate localized high gas pressures due to fragmentation, evaporation and/or decomposition of the impurities. Such pressure may be reduced nondestructively by diffusion of the gases through the metal layer or may cause destructive separation of the metal layer from the substrate over a variably large surface area thereof. Generally, destructive separation of the metal layer manifests as a larger number of blisters which become larger and more numerous with decreasing adhesion.
The adhesion of metal layers deposited electrolytically on nonmetal substrates after premetallization thereof is usually not sufficient to prevent the formation of blisters under thermal stresses caused, for example, by soldering. Therefore, metal layers to be subjected to such thermal stresses have heretofore been produced on nonmetal substrates only by less economical processes requiring more costly apparatus, such as vacuum vapor deposition, cathode sputtering and chemical vapor deposition (CVD).